Nylon 66 density is a little bit different in amorphous and crystalline region at the same temperature. At 25°C amorphous density of nylon 66 is 1.07 g/cm3 and crystalline density of nylon 66 is 1.24 g/cm3
Nylon has variants like nylon 6, nylon 66, nylon 510, nylon 11, nylon 46 etc. In this article, nylon 66 is briefly discussed.
Properties of Nylon 66
Nylon 66 is strong fiber
Melting point 250°C
Nylon 66 has Tg of 50°C (But dry nylon66 has a Tg above 100°C)
Nylon 66 density is 1.31 g/cm3
It has moderate elasticity
Its high tenacity property makes it suitable for industrial use.
The numbers of repeat units along the polymer chains of nylon 6.6 are about 65
Molecular weight of Nylon 66 is about 15 000 g mol–1.
It has 4% moisture regain. These relatively low moisture contents do not allow effective dissipation of accumulated static electric charge, which causes handling and soiling problems. This must be controlled by application of appropriate anti-static chemicals to the fibers, and grounding devices on machinery may be necessary
It is dyed with anionic acid dyes in weakly acidic solution. This process can be considered as a simple anion exchange process in which a counter ion such as acetate, associated with a protonated amino group in the nylon, is exchanged for a dye anion. In many instances, the number of available ammonium ion groups limits the amount of anionic dye taken up by a nylon fiber and direct dyeing of deep shades and blacks becomes impossible.
On the other hand, the amino and carboxylic acid groups do not influence dyeing with disperse dyes. The nylon acts as a solid solvent in which the disperse dye is soluble, so the nylon extracts it from the aqueous bath. This mechanism is identical to that for dyeing cellulose acetate and other synthetic fibers with disperse dyes.
Nylon66 are polyamides and are therefore susceptible to hydrolysis. They are hydrolyzed by boiling solutions of strong acids. For this reason, dyeing is rarely carried out at pH values below 3.0. They are resistant to short treatments with hot alkaline solutions.
Heat setting of nylon materials may be carried out under dry conditions in hot air at 205 °C for nylon 6.6. Alternatively, setting is achieved in steam at 120 or 135 °C. Heat setting is carried out after dyeing unless the fabric has a tendency to form crease marks.
It is very difficult to obtain accurate data for the Tg values of nylons. In particular, absorption of small amounts of water significantly depresses the value of Tg , but it also depends on the extent of drawing and the method of heat setting, and therefore on the degree of orientation and crystallinity. Tg of dry nylon 66 is above 100°C.
How Nylon66 IS Made?
Nylon 6.6, or polyhexamethylene adipamide is obtained by heating the salt of adipic acid and hexamethylene diamine in concentrated aqueous solution in the presence of a little acetic acid.
Once most of the volatile diamine has reacted, water is gradually removed by distillation. Finally, the molten mixture is heated to 280 °C under nitrogen to avoid oxidation.
Fig: Condensation reaction
The final liquid polymer is extruded as a thick ribbon, cooled to solidify it, and cut into chips.
Using the exact 1:1 stoichiometric ratio of the monomers (from the diacid-diamine salt), and forcing the poly-condensation to a high conversion, are important for obtaining the desired high molecular weight of the product.
Addition of a small amount of acetic acid in the polymerization assists in controlling the final molecular weight of the nylon. The added acetic acid functions as a chain stopper by reducing the number of terminal amino groups in the polymer. It is illustrated in the following figure.
The nylon chips are re-melted and the molten polymer extruded to form fine filaments. The melting point of nylon 6.6 is around 250 °C. Melt spinning directly from the polymerization reactor requires very long holding times at a temperature above the melting point.
In melt spinning from nylon chips, the molten polymer is kept under nitrogen. The amount of liquid is so small that the polymer is only molten for a short time.
The hot liquid nylon is filtered to prevent particles of polymer from blocking the holes in the spinneret. It is also important to avoid gas bubbles in the molten polymer since their passage through a spinneret hole gives a broken filament.
After cooling, the nylon filaments are dried and are conditioned in steam before winding onto bobbins.
Then finally the drawing process is carried out. The nylon filaments are drawn by passing them around and between sets of rotating cylinders of increasing speed.
The newly extruded filaments are relatively easy to extend to about four-fold their original length before the stress begins to increase sharply.
Drawing decreases the filament diameter. The elongation causes folded polymer chains to unfold and the chains to slide past each other, gradually increasing the degree of orientation of the polymer molecules along the filament axis. Because of this, there is an increase in crystallinity, giving higher tenacity and reduced elasticity.
What is the Tg of Nylon 66?
The glass transition temperature of completely dry nylon 6.6 is probably over 100 °C, but drops considerably in the presence of small amounts of absorbed water.
Water will form hydrogen bonds with both the hydrogen and oxygen atoms in the amide groups, breaking the existing hydrogen bonds between amide groups in different polymer chains.
The chains therefore have greater mobility, since the water is acting as a plasticizer. In air at room temperature, the filaments are sufficiently plastic for cold drawing.
Salma Hasin Shila, the author of this site completed her BSc. in Textile Engineering (Wet Processing Engineering) from Bangladesh University of Textiles www.butex.edu.bd (BUTEX). She has a passion for textile technology and love to write about it. She wants to share her knowledge to help the students in their studies and businessman & entrepreneurs in their business in making wise decisions fast.